Process for separating phases of different densities and conductivities by electrocoalescence and centrifugation

ABSTRACT

Device for separating a mixture consisting of at least a continuous phase (I) and of at least a conducting disperse phase (II) in the form of particles such as drops in the continuous phase, the two phases having different densities. The device includes at least two substantially cylindrical parts (1, 3) fitted into each other which delimit an annular zone (5), introducing means (6, 7) for introducing the mixture to be separated at the periphery of the outer tubular part (1), arranged so as to communicate a rotational motion to the mixture, means for applying between the two parts (1, 3) a potential difference capable of causing the particles of the disperse phase (II) to coalesce, receiving and decanting means (4), means (8, 9) for discharging the two phases (I) and (II) at least partly separated on account of the differentiated motion of the particles which have coalesced and means for establishing a circulation of the mixture.

This is a division of application Ser. No. 212,762, filed Mar. 15, 1994pending.

FIELD OF THE INVENTION

The present invention relates to a device and to a process forseparating a continuous phase and a conducting disperse phase byelectrocoalescence and centrifugation.

The invention is particularly well suited for desalting and dehydratinga petroleum effluent consisting of an aqueous disperse phase in the formof drops and of a continuous organic phase.

The transportation of crude oil under good technical and economicconditions requires the removal, at least partly, of its aqueous phase.In fact, the transportation and the treating of a volume of aqueousphase involves a loss of pumping or heating energy of the units.Furthermore, the aqueous phase being made up of formation water, itconsists of a possibly highly salted brine of a variable salinityranging for example between 10 g/l and 200 g/l. As it goes throughtransportation, receiving, treating and refining equipments, the aqueousphase may be the origin of salt depositions and of corrosion problemswhich lead to disturbances in the treating chain and particularly inrefining plants. It is therefore increasingly important to have anefficient industrial process for separating the brine from the crudeoil.

BACKGROUND OF THE INVENTION

The prior art describes many devices and processes allowing such aseparation to be performed.

The most simple technique consists in introducing the mixture to beseparated into a chamber whose volume is designed to allow a sufficientresidence time for all the drops forming the dispersed phase to havetime to gather and coalesce at the separation interface between the twophases.

This technique, which is commonly used, involves very bulkyinstallations and possibly relatively long residence times.

An improvement in this technique consists in heating the chamberscontaining the mixture so as to increase the rate of sedimentation ofthe drops and their encounter probability.

Another technique consists in applying an electric field which promotesthe coalescence of the drops of a conducting dispersed phase in arelatively little conducting medium.

In the electrostatic separators applying this principle, breakdownphenomena due to an increase in the electric charge between theelectrodes may sometimes be observed, for example in case of analignment of conducting drops in line with the field.

Other electrode and electric field separator technologies are describedin U.S. Pat. No. 4,601,834, U.S. Pat. No. 1,592,011, SU-1,568,741.

The separation between two phases may be improved by using the effect ofa centrifugal acceleration, as described in patent FR-2,663,238 filed bythe applicant, and by promoting the formation of a film of drops whichhave coalesced on the surface of the inner helical part of the device.

U.S. Pat. No. 4,116,670 describes a device for achieving the coalescenceof drops between two electrodes until drops of a determined size areobtained, the separation of the drops being performed in a centrifugalseparator located after the zone of the device achieving the coalescenceof the drops.

Patent application FR-92/13,360 filed by the applicant describes adevice using the combined effects of the electrocoalescence due to thepresence of an electric field between two electrodes and of thecentrifugal effect resulting from the shape of the electrodes to promotethe phase separation.

It has been discovered, and this is the object of the present invention,that it is possible and advantageous to benefit by the simultaneousaction of an electric field and of a centrifugal effect for separating amixture, by using electrodes of a simple shape, the centrifugal effectbeing due to the tangential introduction of the mixture and to itsflowing inside a revolution volume delimited by the electrodes.

In the text hereafter, the term electrode is understood to be a partbrought to an electric potential.

SUMMARY OF THE INVENTION

The present invention relates to a process for separating a mixtureconsisting of at least a continuous phase and at least a conductingdisperse phase in the form of particles such as drops in the continuousphase, the two phases having different densities. It is characterized inthat the mixture to be separated is introduced into at least one annularzone, said annular zone being delimited by at least two substantiallycylindrical parts fitted into each other, by introducing said mixturetowards the periphery of at least one of the substantially cylindricalparts, so as to communicate thereto a rotational motion in the annularzone, in that a potential difference is applied between the twosubstantially cylindrical parts so as to generate a radial electricfield in the first annular zone to promote the coalescence of thedisperse phase particles and to obtain a separation of the two phases,and in that the two phases, at least partly separated, are collected ina receiving and decanting zone.

The mixture is for example introduced tangentially to the periphery ofat least one of the substantially cylindrical parts.

The mixture to be separated may be circulated in a plurality of annularzones located between coaxial tubular electrodes.

The mixture may be injected simultaneously into several annular zones.

The mixture to be separated may also be circulated successively invarious annular zones.

The mixture to be separated is for example circulated in the firstannular zone delimited by an outer tubular part and an inner tubularpart, the two parts being substantially coaxial, where it undergoes afirst separation during which the particles that have partly coalescedagglomerate towards the periphery of the outer tubular part, the partlyseparated continuous phase passing into a second annular zone delimitedby the inner tubular part and a central electrode, in a globallyopposite direction to the direction of circulation of the mixture in thefirst zone, the remaining particles coalescing during the circulation ofsaid partly separated phase in the second annular zone and agglomeratingon the inner wall of the outer tubular part, the particles which havecoalesced on the inner walls of the outer tubular part and of the innertubular part being drained towards the lower part of the device whilethe rest of the continuous phase is discharged through the pipe.

The process may comprise several mixture separation steps performed inseries, each of these steps allowing a fraction of the disperse phasecontained in the form of drops in the original mixture to be separated.

A continuous potential difference is for example applied between theparts or electrodes delimiting an annular zone to generate an electricfield.

An alternating potential difference or a periodic-varying potentialdifference of constant sign may also be applied between the parts orelectrodes delimiting an annular zone to generate an electric field.

The peak value of the electric field established within an annular zonemay range between 3.10⁴ and 6.10⁵ V/m.

The average velocity of flow of the mixture to be separated in anannular space preferably ranges between 1 and 10 m/s.

The disperse phase particles which have partly coalesced and which leaveeach annular zone are for example separated from the continuous phase bystatic centrifugation.

The invention further relates to a device for separating a mixtureconsisting of at least a continuous phase and at least a conductingdisperse phase in the form of particles such as drops in the continuousphase, the two phases having different densities. It includes at leasttwo substantially cylindrical parts fitted into each other anddelimiting an annular zone between each other, introducing means forintroducing the mixture to be separated at the periphery of at least oneof the parts arranged so as to communicate a rotational motion to themixture, means for applying between the two substantially cylindricalparts a potential difference capable of causing the disperse phaseparticles to coalesce, receiving and decanting means, means fordischarging the two phases at least partly separated on account of thedifferentiated motion of the particles which have coalesced and meansfor establishing a circulation of the mixture.

The means for discharging the partly separated continuous phase andconducting disperse phase are located in different places.

The substantially cylindrical parts comprise for example an outertubular part and an inner part, said parts being coaxial.

The inner part may be a tubular part forming a pipe through which thecontinuous phase is discharged.

The inner cylindrical electrode is covered with an insulating materialsuch as a polymer.

The device may include a plurality of coaxial parts performing thefunction of electrodes delimiting a plurality of annular zones and meansfor applying between any two adjacent electrodes delimiting an annularzone a potential difference capable of causing the drops of the disperseconducting phase to coalesce.

The device may include means for leading in the mixture to be separatedin parallel into the annular zones, said means being located on theperiphery of each of the annular zones and according to a substantiallytangential direction to the periphery of each of the annular zones, andat the same end of the device.

The outlet of an annular zone communicates for example with the inlet ofthe next adjacent annular zone so that the mixture to be separated runsthrough the successive annular zones in series, one after the other.

A grid may be arranged close to the inner wall of the inner part so asto stabilize the film consisting of drops which have coalesced on saidwall.

The receiving and separating means may comprise a conical part insidewhich the phase formed by the particles which have coalesced and carriedalong by the continuous phase is separated by hydrocycloniccentrifugation.

The device may include a means for detecting the interface between thepartly separated phases, so as to maintain the interface level betweentwo given values.

The process and the device according to the invention are particularlywell suited to the separation of a mixture such as a petroleum effluentcontaining a disperse aqueous phase and an organic continuous phaseand/or to the desalting of crude oil comprising the admixture ofrelatively little salted water.

One advantage of the present invention is to minimize drop chainingphenomena between two neighbouring or adjacent electrodes since thedrops are kept away from one of the two electrodes under the effect ofthe centrifugal force. Breakdown risks due to the alignment of the dropsbetween two successive electrodes are therefore reduced.

Another advantage is to increase the separation efficiency through theoptimum use of the treating volume where the electrocoalescencephenomenon and the centrifugal effect are combined.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be clearfrom reading the description hereafter, with reference to theaccompanying drawings in which:

FIGS. 1A and 1B diagrammatically show a separation device according tothe invention and a topview of the mixture tangential supply pipes,

FIGS. 2A and 2B diagrammatically show a device allowing a finerseparation of the mixture to be achieved,

FIGS. 3A and 3B schematize embodiments of the device including severalannular zones of separation by electrocoalescence and centrifugation, inparallel and in series,

FIGS. 4A and 4B show a variant of the device including spiral-shapedannular separation zones,

FIG. 5 schematizes a device including a hydrocyclonic separation zonesetted after the zone of separation by electrocoalescence andcentrifugation,

FIG. 6 shows a separation device consisting of two separation elements,

FIG. 7A schematizes a possible mode of combination of several devicesaccording to the invention, and

FIG. 7B is a cross-section of the device along the axis B--B of FIG. 7A,and FIG. 7C is a topview of the assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device which is described hereafter allows a mixture containing aconducting disperse phase in the form of drops and a continuous phase tobe separated by using the combined effects of electrocoalescence and ofcentrifugation obtained by introducing the mixture tangentially into aseparation chamber, so that the disperse phase drops which havecoalesced gather towards the chamber periphery, which leads to an atleast partial separation of the two phases.

FIG. 1A shows an example of such a device which includes a chamber 1having an axis A and containing a substantially cylindrical part 2performing the function of an outer tubular part in which an innertubular part 3 is arranged, the two parts being preferably coaxial, anda decanting and receiving zone 4 extending the cylindrical zone 2. Thelength of the inner tubular part 3 is preferably at most equal to thelength of the cylindrical zone 2, the cylindrical part 2 of the chamberand the inner tubular part 3 delimiting an annular zone 5 of separationby electrocoalescence and centrifugation. It further includes two pipes6, 7 (FIG.1B) for introducing the mixture to be separated, arrangedtangentially and located in the upper part of the chamber. The chamberalso comprises pipes 8, 9 for discharging respectively the phaseconsisting of drops having coalesced and the continuous phase, thedischarge pipe 9 corresponds in this embodiment to the hollow section ofthe inner part 3.

Chamber 1 is closed in its upper part by a cover 10 in which inner part3 is fitted. Cover 10 is provided with an opening communicating pipe 9with an outer discharge tube T. Cover 10 and inner part 3 are insulatedelectrically by an insulating joint 11.

Chamber 1 and inner part 3 are brought into contact with the terminalsof a device such as a high-voltage electric generator G supplied throughthe mains and piloted by a function generator, allowing the inner part 3to be brought to a given potential value while chamber 1 is preferablygrounded.

In the receiving zone 4 of the chamber, a conical part 12 is arranged tocreate a zone referred to as a calm zone 13 where an interface betweenthe coalesced disperse phase and the continuous phase may be stabilizedby means of a level detector 14 well known to specialists, connected todischarge pipe 8 through a valve V, allowing the rate of flow of thecoalesced drops to be controlled in order to maintain the interfacebetween the coalesced drops and the continuous phase at a given distancefrom the zone where the separation by electrocoalescence andcentrifugation is performed. Upon leaving the annular zone, thecontinuous phase, whose density is lower than that of the dispersephase, is gathered around the central part and passes inside this partthrough pipe 9 and is thereafter discharged through tube T. During theextraction of the continuous phase through pipe 9, the conical part 12allows zones of turbulence and of redispersion of the at least partlyseparated phases to be limited.

The object of device 14 is to maintain the interface level between twogiven values, one being located for example above pipe 8 and the other,below the zone where the separation by electrocoalescence andcentrifugation is performed.

In case the mixture consists of a strongly conducting brine, the innerpart 3, performing the function of an electrode brought to a highvoltage value, is covered with a layer of insulating polymer throughmethods known to the man skilled in the art. The choice of theinsulating material and of its thickness is well known to specialistsand depends on the mixture to be separated.

This embodiment allows breakdown phenomena resulting notably from localshort circuits when the value of the electric field is raised to beavoided.

The potential difference applied between the inner part and the chamberdefends on the individual case.

A continuous electromotive force (emf) is applied between two partsdelimiting an annular zone when a mixture is to be separated in whichthe conducting disperse phase has the form of a little concentratedemulsion within a little conducting continuous phase.

In order to separate mixtures consisting of relatively concentratedemulsions, it is more advantageous to use variable voltage sourcesgenerating an alternating voltage or a pulsed or impulsive electromotiveforce, constantly of equal sign, for example.

The frequency value and the intensity of the electromotive force used isselected according to the nature of the mixture to be separated and tothe type of parts performing the function of electrodes in the device.

The frequency values range for example between 0.1 and 100 Hz,preferably between 50 and 60 Hz. In order to improve the separation, itis possible to use a frequency higher than 100 Hz but less than 1000 Hzin order to avoid a redispersion of the coalesced drops. The intensityof the electric field ranges for example between 3.10⁴ and 6.10⁵ V/m.

The average circulation velocity of the mixture is choosen to obtain acentrifugal force allowing the coalesced drops to be separated from theoriginal mixture, the value of this velocity in an annular zone rangingpreferably between 1 and 10 m/s. A centrifugal acceleration ranging forexample between 10 g and 500 g is obtained thereby, g being thegravitational acceleration.

The residence time of the mixture in the chamber depends on the desireddegree of separation of the mixture and on the stability of the mixtureto be separated. It ranges for example between several ten seconds andseveral minutes.

One possibility of use of the device according to the invention consistsin introducing the mixture to be separated, containing the continuousphase and the disperse conducting phase in the form of drops, throughpipes 6, 7, the density of the disperse phase being in this instancehigher than the density of the continuous phase. The mixture isintroduced at a sufficient speed to communicate thereto a rotationalmotion promoting the separation of the two phases. A potentialdifference is applied between the inner part 3 and chamber 1, thusperforming the function of electrodes, this tension generating a radialelectric field substantially perpendicular to the electrodes in theannular zone 5 of circulation of the mixture to be separated. Under thecombined action of the electric field and of the centrifugal force, thedisperse phase drops coalesce and agglomerate progressively while themixture flows through the annular zone. While the mixtures runs, thecoalesced disperse phase drops of higher density, more subjected to thecentrifugal force resulting from the rotational motion, are displacedtowards the periphery of the device, i.e. towards the periphery ofchamber 1 where they agglomerate and form for example a film on theinner wall 2a of the chamber of the cylindrical zone 2. The mixture,partly separated when leaving the annular zone, flows thereafter throughzone 4 of the chamber. The coalesced drops flow for example in the formof a film along the cylindrical zone 2 under the effect of their ownweight and/or through a carry-over effect and they are collectedthereafter in the receiving zone 4 of the chamber and discharged throughpipe 8.

The continuous phase of lower density is collected in the upper part ofreceiving zone 4, above conical part 12, and discharged thereafterthrough pipe 9 and tube T.

FIGS. 2A, 2B, 3A and 3B show variants of the device according to theinvention allowing a more advanced separation of the mixture to beobtained.

The device shown in FIG. 2A includes a cylindrical electrode 15 arrangedcoaxially to the inside of the pipe 9 for discharging the continuousphase. This electrode 15 is linked electrically to chamber 1. Theintensity of the radial electric field prevailing inside pipe 9 isdetermined by the inside diameter of tubular part 3 and by the outsidediameter of electrode 15. This field allows the disperse phase dropscarried over into the continuous phase after the separation stageachieved in the annular zone 5 to coalesce with each other and, underthe effect of the centrifugal force, to be separated from the continuousphase by migration towards the inner wall of tubular part 3 on whichthey form for example a film and flow thereafter downwards towards thebottom of the chamber under the effect of gravity. An advancedseparation of the last drops of the conducting disperse phase carriedover into the continuous phase at the outlet of the annular zone 5 isobtained thereby.

As the concentration of the disperse phase drops carried over with thecontinuous phase is lower than that of the drops of the initial mixture,the spacing between electrodes 3 and 15 is preferably less than thedistance between the tubular part 3 and chamber 1 so as to have a moreintense electric field in pipe 9 than the field in annular zone 5.

In order to facilitate the draining of the drops along the inner wall 3aof tubular part 3, the drops exhibiting a countercurrent flow withrespect to the continuous phase discharge, the inner wall 3a of theelectrode may be grooved. It may thus comprise vertical grooves or becovered with hydrophilic fibers or with a grid performing the samefunction as the grooves; these various examples are not shown in thefigure. A zone of limited shear is obtained thereby, which allows thefilm formed by the coalesced drops to flow while minimizing redispersionrisks.

Pipe 9 may include a non conducting helical part 16 (FIG.2B) allowingthe rotational motion of the mixture as leaves annular zone 5 and enterspipe 9 to be enhanced. This part is for example positioned from thelower end of pipe 9, and its length may be equal to part of the lengthof the pipe or to the total length thereof.

In the variant shown in FIG. 3A, a finer separation is achieved bypassing the mixture through several coaxial annular zones.

Chamber 1 includes three coaxial annular zones or spaces 5, 17 and 18delimited respectively by the cylindrical part 2 of chamber 1 and by aconcentric inner part 19 performing the function of an electrode, byelectrode 19 and by the tubular part 3, and by tubular part 3 and acylindrical central electrode 20 located inside discharge pipe 9 andinsulated from chamber 1 by an insulating part 21, preventing therebyshort circuits between electrode 20 and the cylindrical part 2 of thechamber. Electrodes 19, 20 are electrically connected together andlinked to the high-voltage terminal of generator G while the tubularpart 3 and chamber 1 are connected to the other generator terminal. Twoadjacent electrodes are brought thereby to a different electricpotential so as to generate a substantially radial electric field in theannular zones 5, 17 and 18. Electrode 19 is linked electrically tochamber 1 by parts 22 such as positioning strips, this connection alsoallowing electrode 19 to be held up in chamber 1. The mixture to beseparated is introduced as in the embodiments described above throughthe tangential pipes 6, 7 and enters a distribution zone 23 located inthe upper zone of chamber 1 from which the mixture is distributed in theannular zones 5 and 17 and runs in a rotational motion on account of itstangential delivery into the chamber. Similarly to the previousexamples, the drops which have coalesced in zones 5 and 17 aredischarged at least partly through pipe 8, while the mixture consistingmainly of the continuous phase from zones 5 and 17 passes into theannular zone 18 where it undergoes a final separation stage through thecombined action of the centrifugal force and of the electric fieldprevailing in this zone. The last drops coalesce and run along the innerwall of part 3 before they are discharged through pipe 8. The continuousphase is discharged through pipe 9 on account of its density which islower than that of the mixture present in the annular zone 5.

The rotational motion may be enhanced by setting a helical part,identical to part 16 shown in FIG. 2A, inside pipe 9, around electrode20.

The spacing between the electrodes is choosen according to theconcentration of the disperse phase drops in the mixture to be separatedor in the partly separated continuous phase. The spacing betweenelectrodes 3 and 20 is therefore shorter than that between electrodes 2and 19 or between electrodes 3 and 19.

This embodiment using the parallel feed of several annular zones allowsthe intensity of the electric field in the various annular zones to bevaried, the electric and hydrodynamic conditions being equal.

In the embodiment of FIG. 3B, the chamber includes several annular zonesso arranged that the outlet of an annular zone communicates with theinlet of the next annular zone. The mixture runs therefore through thezones in series, one after the other.

The device comprises a concentric cylindrical inner part 24 connected tothe upper cover 10 of chamber 1 and delimiting therewith a first annularzone 25, a second part having the shape of a cylindrical tube closed atthe lower end thereof 26, located inside part 24 and concentric thereto,forming a second annular zone 27 with part 24, the outlet of the annularzone 25 communicating with the inlet of zone 27. The tubular part 3located inside tube 26 forms a third annular zone 28 positioned afterzone 27 and, finally, a central electrode 29 located inside tubular part3 and connected to the lower end of tube 26 delimits a fourth annularzone 30 positioned after zone 28. The lower end of tube 26 opens intochamber 1 so as to form a zone of discharge of the coalesced drops. Itis provided with a device 31 for detecting the interface level,identical to device 14, connected to a flow-control valve V described inFIG. 1A and located on a discharge pipe 32.

The cylindrical parts 3 and 24 are connected to the high-voltageterminal of generator G, whereas chamber 1 and parts 26, 29 areconnected to the other terminal of the generator so as to create in theannular zones 25, 27, 28 and 30 an electric field promoting thecoalescence of the disperse phase drops.

The mixture to be separated introduced through the tangential pipes 6, 7runs through zones 25, 27, 28 and 30 in series. As it flows through thevarious annular zones, the combined effect of the electric fields and ofthe centrifugal force leads to the coalescence of the disperse phasedrops and to the increasingly finer separation of the coalesced dropsfrom the continuous phase. The disperse phase drops coalesce and migratetowards the periphery of the annular zones, and run thereafter along thewalls of the parts delimiting these annular zones, for example in theform of a film, towards the lower end of chamber 1 where they aredischarged through pipes 8 and 32.

This embodiment is more particularly suited for industrial desaltingoperations conducted in several steps going from a coarse separation toa finer separation. Zones 25 and 27 are called coarse separation zones,the coalesced salt-saturated drops are discharged through pipe 8. Thecontinuous phase containing drops from the disperse phase which have notcoalesced, or insufficiently to be separated, passes into zone 28 whereit is mixed again with little salted dilution water introducedtangentially through delivery pipe 33. The new mixture formed therebypasses then into annular zones 28 and 30 where it is progressivelyseparated and thus desalted. The drops which have coalesced in zones 28and 30 are collected in the lower part of tube 26 and discharged throughpipe 32.

The flow rates of the pipes 8 and 32 for discharging the coalesced dropsare preferably subjected to a level control by means of the devices 14,31 known to specialists.

The device shown in FIG. 4A comprises two spiral-shaped inner partsperforming the function of electrodes 34, 35 fitted into one another andcoaxial to insulating chamber 38. The two parts are brought to adifferent electric potential and delimit thereby two annular zones 36,37 (FIG. 4B) in which a radial electric field of equal intensityprevails. The winding of these parts is so achieved that electrodes 34,35 are not in contact.

The mixture to be separated is introduced tangentially to the chamberthrough pipes 6, 7 which open respectively into the annular zones 36,37. The winding of the parts and the tangential introduction of themixture gives the mixture a high rotational speed. Under the combinedeffect of the electric field and of the centrifugal force, the dispersephase drops tend to coalesce and to migrate towards the periphery of theannular zones, the inner wall of electrode 34 for zone 36 and the innerwall of electrode 35 for zone 37. The coalesced drops form for example afilm on the surface of the electrodes which is driven by gravity towardsthe lower end of chamber 1 in zone 39, the film being thereafterdischarged through a pipe 40 located at the lower end of chamber 1. Thepart of the continuous phase depleted in disperse phase on account ofthe centrifugal effect runs through the annular zones 36, 37, thenthrough zone 42 delimited by parts 34 and 35, and is finally dischargedthrough pipe 43 located in the upper central part of the chamber.

This type of configuration promotes the continuous draining of thecoalesced particles towards zone 39 called a "calm zone" and therebyreduces the risks of re-entrainment of the coalesced drops with thecontinuous phase.

In the embodiment of FIG. 5, the discharge of the phases partlyseparated after passing through an annular zone is performed moreefficiently by means of a hydrocyclonic centrifugal effect.

The zone 4 of chamber 1 in FIG. 1 is replaced by a conical-shaped part44 prolonged by a discharge pipe 45. The conical shape of part 44 allowsthe mixture situated in this zone, mainly consisting of coalesced drops,to keep its rotational motion until it is extracted through pipe 45. Theproportions of part 44 are identical or similar for example to those ofthe hydrocyclones known to specialists.

FIG. 6 shows an embodiment of the device according to the inventionwhich combines in series two devices I and II, such as those shown inFIG. 5, more particularly suited for the two-step separation of amixture.

The desalting of a "crude" or petroleum effluent consisting of acontinuous phase such as oil and of an aqueous phase in the dispersedform of drops may be achieved thereby. Devices I and II are connected toeach other by lines and valves described hereunder.

The crude to be separated is introduced through pipe 46 and may be mixedwith water coming for example from device II by means of a mixing valve47. The new water-crude mixture enters device I tangentially throughpipe 48 where it is separated at least partly, the salt-saturated dropswhich have coalesced being discharged through pipe 45 located in thehydrocyclonic part of the device and the continuous phase beingdischarged through a pipe 49 corresponding to the inside of the tubularpart 3 and connected to the delivery pipe 50 of device II. Thecontinuous phase still containing drops which have not or insufficientlycoalesced in device I passes into the second device II through a pipe 50fitted with a mixing valve 51. It is thereafter mixed with water comingfrom a source of water with a very low salt content, not shown in thefigure, through a pipe 52 and valve 51. This water supply allows thedissolved salts to be diluted. The new mixture enters device II in orderto achieve the separation of the residual drops and of the new waterdrops coming from the source. The separation is performed identically tothe embodiments described above. The continuous phase is dischargedthrough the pipe 53 corresponding to the hollow portion of tubular part3 and the coalesced drops are discharged through pipe 45.

In a preferred embodiment, the operation of the separators is soadjusted that the electric field is lower in the first device I than inthe second because of the higher concentration and of the greaterdiameter of the disperse phase drops in the mixture to be separated, andthat the draw-off ratio is low enough to limit the carry-over of the oilin the aqueous phase discharge pipe. The value of the electric fieldintensity prevailing in the annular zone 5 of device II is higher thanthat of device I because of the concentration and the smaller diameterof the disperse phase drops, the mixture or brine being less salted. Thedraw-off ratio in this device must be high enough to prevent thecarry-over of the aqueous phase by the continuous phase dischargedthrough pipe 53.

In case the pressure of the mixture introduced in the device I is toolow, a pump P located on pipe 50 is necessary to supply a sufficientinjection rate of the mixture in device II.

In case the water-rich phase extracted from device II through pipe 45 isrecirculated towards the feed point of device I, the pump P is used forequalizing the pressure of the mixture from the pipe 45 of device IIwith the pressure of the mixture circulating in pipe 46.

FIGS. 7A, 7B and 7C show an example of a parallel association of severaldevices described previously, adapted to industrial treating capacities.FIG.7A shows the lay-out of the separators arranged in vertical bundlescomprising 8 elements (FIGS. 7B, 7C) fed by central pipe 54. This pipeopens into a dispatcher 55 for introducing identically and tangentiallythe mixture to be separated into each of the annular zones 5 of theseparator elements. The oil or continuous phase is discharged throughvertical pipes 9 which join in a shunt 56. The aqueous phase consistingmainly of the coalesced particles is discharged through pipes 57 and 58while the level is controlled by means of a device 59 identical todevices 14 and connected in the same way to a valve V (FIG. 1A).

Without departing from the scope of the invention, the tubular innerpart 3 may be replaced by a solid part performing the same function. Thecontinuous phase is then discharged through a pipe arranged in thereceiving zone and located above the pipe 8 for discharging the drops ofthe phase which have coalesced.

Part 3 may be brought into rotation with respect to chamber 1. Theadvantage of this rotational motion is to enhance the rotational motionof the mixture in the zone formed by pipe 9 and to increase thecentrifugal force in annular zone 5. The direction of rotation of part 3with respect to chamber 1 is identical to the direction of delivery ofthe mixture and to the direction of the motion it follows in zone 5.

In order to improve the separation and to obtain a more advancedseparation of the mixture, also called a secondary separation, varioustechniques known to the man skilled in the art may be used, such as theinterception of the drops, the adsorption of these drops on ahydrophilic porous material, . . .

Of course, the process and the device which have been described by wayof non limitative examples may be provided with various modificationsand/or additions by the man skilled in the art without departing fromthe scope of the invention.

We claim:
 1. A process for separating a fluid mixture containing atleast a continuous phase (I) and of at least a conducting dispersedphase (II) comprising particles, the two phases having differentdensities and comprising liquid drops dispersed within the continuousphase, wherein the mixture to be separated is introduced into a devicehaving at least one annular zone delimited by two substantiallycylindrical parts, one part being fitted into another part, and saidmixture being introduced along a periphery of the one part to produce arotational motion of the mixture in the annular zone around the one partand to impart a centrifugal force to the mixture, simultaneously apotential difference is applied between said two substantiallycylindrical parts to create a radial electric field in the annular zonefor causing the particles of the dispersed phase (II) to coalesce and toat least partially separate from the continuous phase due to thecentrifugal force in the annular zone, the centrifugal force causingseparation of the coalesced particles of the dispersed phase at leastpartially inside the annular zone, and the continuous phase and thedispersed phase being collected separately outside of the annular zone.2. A process according to claim 1, wherein the mixture is introducedtangentially to a periphery of at least one of the substantiallycylindrical parts.
 3. A process as claimed in claim 1, wherein themixture to be separated is circulated in a plurality of annular zones,each annular zone being located between coaxially arranged tubular partsto promote separation of the continuous and dispersed phases; apotential difference being applied to each annular zone to create aradial electric field therein for promoting coalescence of theparticles.
 4. A process according to claim 3, wherein the mixture isinjected simultaneously into the plurality of annular zones.
 5. Aprocess according to claim 3, wherein the mixture is circulatedsuccessively in a series of the annular zones.
 6. A process according toclaim 3, wherein the mixture to be separated is circulated in a firstannular zone, delimited by an outer tubular part and an inner tubularpart, said tubular parts being substantially coaxial, in the firstannular zone the mixture undergoes a first separation during whichparticles which have partly coalesced agglomerate towards a periphery ofthe outer tubular part, the partly separated continuous phase thenpasses into a second annular zone delimited by the inner tubular partwhich comprises a central electrode, in an opposite direction to adirection of circulation of the mixture in the first annular zone, theremaining dispersed particles coalescing during the circulation of saidpartly separated continuous phase in the second annular zone toagglomerate on an inner wall of the inner tubular part, and theparticles coalesced on the inner walls of the outer tubular part and ofthe inner tubular part, respectively, being drained as liquid towards alower portion of the device while the continuous phase separated fromthe dispersed phase is discharged though a pipe located at an upperportion of the device.
 7. A process according to claim 1, comprisingseveral separation steps conducted in series, each of these stepsallowing a fraction of the disperse phase (II) in the form of liquiddrops in the mixture to be separated.
 8. A process according to claim 1,wherein a continuous potential difference is applied between the partsdelimiting the annular zone so as to generate the electric field.
 9. Aprocess according to claim 1, wherein an alternating potentialdifference is applied between the parts delimiting annular zone so as togenerate the electric field.
 10. A process according to claim 1, whereina periodic-varying potential difference of constant sign is appliedbetween the parts delimiting the annular zone.
 11. A process accordingto claim 1, wherein the peak or maximum value of the electric fieldestablished within the annular zone ranges between 3.10⁴ and 6.10⁵ V/m.12. A process according to claim 1, wherein an average velocity of flowof the mixture in the annular zone ranges between 1 and 10 m/s.
 13. Aprocess according to claim 1, wherein the dispersed phase particleswhich have partly coalesced and leave the annular zone are furtherseparated from the continuous phase by static centrifugation in anotherannular zone.
 14. A process accordinq to claim 1, for the separation ofa petroleum effluent wherein said mixture comprises the petroleumeffluent containing an aqueous phase as the dispersed phase within apetroleum containing continuous phase.
 15. A process according to claim14, for effecting the desalting of crude oil, wherein said mixturecomprises an admixture containing relatively little salt water.